The present invention relates to optical amplifiers within wavelength division multiplexed optical communications systems. More particularly, the present invention relates to an optical amplifier system and a method for optical amplification within a wavelength division multiplexed optical communications system wherein a first plurality of optical signal channels propagate in a first direction and a second plurality of optical signal channels that are interleaved with the first plurality propagate in a second direction opposite to the first direction.
Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. Wavelength division multiplexing (WDM) is used in such fiber optic communication systems to transfer a relatively large amount of data at a high speed. In wavelength division multiplexing, multiple information-carrying signals, each signal comprising light of a specific restricted wavelength range, may be transmitted along the same optical fiber.
In this document, the individual information-carrying lights of a WDM system are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannels.xe2x80x9d The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as either a xe2x80x9ccomposite optical signalxe2x80x9d or simply as a xe2x80x9cplurality of optical channelsxe2x80x9d. Each information-carrying channel actually comprises light of a certain range of physical wavelengths within a band. However, for simplicity, an individual channel is often referenced to a single wavelength, xcex, at the nominal center of the band. A plurality of such channels (i.e., a composite optical signal) are often denoted by a set of indexed wavelengths, such as xe2x80x9cxcex1-xcexnxe2x80x9d, or xe2x80x9cxcex1, xcex2, xcex3, . . . xe2x80x9d, etc.
Strictly speaking, a multiplexer is an apparatus which combines separate channels into a single wavelength division multiplexed composite optical signal and a de-multiplexer is an apparatus that separates a composite optical signal into one or more subsets of component channels. However, since many multiplexers and de-multiplexers ordinarily operate in either sense, the single term xe2x80x9cmultiplexerxe2x80x9d is usually utilized to described either type of apparatus. Because of the nature of the present invention, however, the precise usage of the terms multiplexer and de-multiplexer is adhered to in this documentxe2x80x94that is, as used in this document, a xe2x80x9cmultiplexerxe2x80x9d (MUX) combines channels but does not operate in the reverse sense so as to separate channels and a xe2x80x9cde-multiplexerxe2x80x9d (DEMUX) separates channels but does not operate in the reverse sense so as to combine channels. An apparatus which can perform either channel separation or channel combining is referred to in this document as a xe2x80x9cchannel separatorxe2x80x9d.
As the terms are used in this document, either a multiplexer or a channel separator may perform an interleaving function and either a de-multiplexer or a channel separator may perform a de-interleaving operation. An interleaving operation occurs when a first composite optical signal comprising a first plurality of optical channels is multiplexed together with a second composite optical signal comprising a second plurality of optical channels, wherein the first plurality of channels and the second plurality of channels are interleaved with one another. It is to be understood that the above stipulation that xe2x80x9cthe first plurality of channels and the second plurality of channels are interleaved with one anotherxe2x80x9d means that the wavelengths (or frequencies) of the first plurality of optical channels are interleaved with the wavelengths (or frequencies) of the second plurality of optical channels. A de-interleaving operation is the opposite of an interleaving operation. Multiplexers, de-multiplexers and channel separators that perform interleaving or de-interleaving operations are herein referred to as interleaved channel multiplexers, interleaved channel de-multiplexers and interleaved channel separators.
An apparatus that performs a de-multiplexing operation is referred to herein as an xe2x80x9cmxc3x97n de-multiplexerxe2x80x9d where m is an integer representing the number of input ports, n is an integer representing the number of output ports and nxe2x89xa7m. An apparatus that performs a multiplexing operation is referred to herein as an xe2x80x9cjxc3x97k multiplexerxe2x80x9d where j is an integer representing the number of input ports, k is an integer representing the number of output ports and jxe2x89xa7k. Channel separator apparatuses are referred to herein as an xe2x80x9cixc3x97j channel separatorxe2x80x9d apparatuses where i is an integer representing the number of a first logical or physical group of ports, and j is an integer representing the number of a second logical or physical group of ports, wherein optical signals may propagate between the first and second groups but not between one port and another within an individual group.
It is desirable, within many fiber optic wavelength division multiplexed optical communications systems, for optical signals to be transmitted bi-directionallyxe2x80x94that is, such that one or more first optical signals comprising a first wavelength or a first plurality of wavelengths are propagated in one direction whilst one or more second optical signals comprising either a second wavelength or a second plurality of wavelenghs are propagated in the opposite direction. Where optical signals propagate within a long transmission line, it is frequently necessary to amplify the bidirectional signals at intermediate points. Since the construction of most optical amplifiers only permits unidirectional optical transmission through the amplifier, it is thus necessary to interrupt the transmission path, route the two counter-propagating signals unidirectionally through the optical amplifier, and then return them to the transmission path to continue in their original, opposite directions of propagation. The temporary conversion of the counter-propagating signals into a combined unidirectional signal through the optical amplifier enables a single amplifier to be used, thereby saving expense and avoiding differences of amplification.
A prior-art bi-directional amplifier apparatus of this type is illustrated in FIG. 1. The prior-art apparatus 100 shown in FIG. 1 comprises a series of wavelength-selective devices 10, 12, 14 and 16, each having four ports designated P1, P2, P3 and P4, respectively. Each device comprises two graded-index one quarter pitch lenses disposed end-to-end with an optical bandpass filter sandwiched between their juxtaposed ends. Reference numbers utilized in FIG. 1 have the suffix xe2x80x9cLxe2x80x9d or xe2x80x9cRxe2x80x9d to identify whether it is at the right hand side or the left hand side, as shown. Thus, the left hand lenses of devices 10, 12, 14 and 16 are designated 10L, 12L, 14L and 16L, respectively, and the right hand lenses are designated 10R, 12R, 14R and 16R, respectively. The lenses are arranged so that light beams from each first port P1 will be collimated by the left-hand lens to illuminate substantially the whole of the corresponding bandpass filter and refocussed by the right-hand lens to couple into the opposite port P4xe2x80x94and vice versa. Likewise, light beams from the second port P2 will be collimated as they pass through the filter and refocussed to couple into the opposite third port P3xe2x80x94and vice versa. Where the collimated light beams from one port are reflected by the filter, they will be refocussed by the same lens but couple to the adjacent port.
Whereas the lenses are identical, each of the bandpass filters, designated 10F, 12F, 14F and 16F, respectively, will transmit a different band of wavelengths. The passbands of the bandpass filters 10F, 12F, 14F and 16F are designated xcex92, xcex94, xcex96 and xcex9N, respectively. Each bandpass filter will pass light beams having a wavelength within its passband to couple to the opposite port and reflect light beams having a wavelength outside its passband so that they couple to the adjacent port. Light which passes through or is reflected from the optical band pass filter at the proper angle is refocused by the lens to a point sufficiently small to transfer the light from the lens couples into the fiber with minimum loss.
The first port P1 and the third port P3 of the first device 10 are connected to respective ends of first and second sections 18L and 18R of an optical fiber transmission line. The transmission line supports propagation of two groups of wavelength-division multiplexed (WDM) optical signals, one in each direction. In the drawing, even-numbered optical signals, designated as xcex2 through xcexN, propagate from right to left in the transmission line and odd-numbered optical signals xcex1 through xcexM propagate from left to right. It should be noted that optical signal xcex2 will have a wavelength within the passband xcex2 of filter 10F, optical signal xcex4 will have a wavelength within the passband xcex4 of filter 12F, and so on. The four devices 10, 12, 14 and 16 are xe2x80x9cchainedxe2x80x9d in that the second and fourth ports P2 and P4 of each of the first three devices 10, 12 and 14 are connected to the first and third ports P1 and P3, respectively, of the succeeding device. The second port P2 and fourth port P4 of the final device 16, however, are connected to the input port IN and output port OUT, respectively, of a unidirectional optical amplifier or other signal treatment equipment 20. The interconnections 22 between the various ports of the components may be optical fiber or any other suitable means.
In operation, the chain of four port devices 10-16 extract the two groups of WDM signals from the respective optical fiber sections 18L and 18R, and convert them into a single unidirectional set of signals which is supplied to the input port IN of the amplifier 20. When the unidirectional set of signals leave the output port OUT of the amplifier 20, the chain of devices convert them back into the original two groups of WDM signals and return each group to the other of the optical fiber sections 18L and 18R to continue propagating along the transmission line in the original direction.
Although the prior-art apparatus shown in FIG. 1 appears to be capable of performing its intended function, it possesses several drawbacks as a result of the potentially large number of optical filters and other components which must be utilized. The number of filters required in the prior-art apparatus 100 (FIG. 1) is equivalent to the number of channels in the set of channelsxe2x80x94of the two counter-propagating setsxe2x80x94with the lesser number of channels. The potentially large number of filters utilized in the apparatus 100 leads to un-necessary complexity and bulk and to fabrication inefficiency and high fabrication cost. Since each optical signal interacts withxe2x80x94that is, passes through or is reflected fromxe2x80x94each filter twice in its passage through the apparatus 100, the insertion loss may be great if several filters are utilized. Further, each one of the plurality of filters 10F, 12F, 14F, etc. must be pre-selected for optical performance characteristics, since each filter operates independently of all the others. Finally, filters of the type comprising the apparatus 100 are generally not available for inter-channel spacings less than approximately 100 GHz. This precludes the use of the prior-art apparatus for utilization with optical communications systems comprising more densely spaced channels.
Accordingly, there is a need for an improved bi-directional optical amplifier system. Such a system should have a minimal number of channel separation and re-combining stages so as to yield improved optical throughput, reduced size and less costly, more efficient fabrication than the conventional filter-based apparatus. Preferably, the improved apparatus should be capable of operating with inter-channel spacing of less than 100 GHz.
Accordingly, to address the above-described needs and drawbacks of the prior-art, an improved bi-directional optical amplifier system and a method for bi-directional optical amplification are herein disclosed. In a first embodiment, a bi-directional amplifier system in accordance with the present invention comprises a 2xc3x972 interleaved channel separator optically coupled to both a first and a second bi-directional fiber optic communications line and an optical amplifier optically coupled to the 2xc3x972 interleaved channel separator. A second embodiment of a bi-directional amplifier system in accordance with the present invention comprises a first and a second 1xc3x972 interleaved channel separator optically coupled to a first and a second bi-directional fiber optic communications line, respectively, an optically isolating 2xc3x971 interleaved channel multiplexer and an optically isolating 1xc3x972 interleaved channel de-multiplexer optically coupled to both of the 1xc3x972 interleaved channel separators and an optical amplifier optically coupled to both the optically isolating 2xc3x971 interleaved channel multiplexer and to the optically isolating 1xc3x972 interleaved channel de-multiplexer.
An interleaved channel separator suitable for use within a bi-directional amplifier system in accordance with the present invention comprises a plurality of ports, at least one of a first lens optically coupled to at least a first of the plurality of ports, at least one of a second lens optically coupled to at least a second of the plurality of ports, a polarization beam splitter optically coupled to the lenses and at least two nonlinear interferometers optically coupled to the polarization beam splitter. One form of an optically isolating interleaved multiplexer or de-multiplexer suitable for use within a bi-directional amplifier system in accordance with the present invention comprises a plurality of polarizing optical ports, a first and a second polarization beam splitter, a first non-reciprocal optical rotator and a first reciprocal optical rotator optically coupled between the first and the second polarization beam splitters, a second non-reciprocal optical rotator and a second reciprocal optical rotator optically coupled to the second polarization beam splitter, and a non-linear interferometer optically coupled to the second polarization beam splitter.
A method of bi-directional optical amplification of in accordance with the present invention comprises the steps of: (a) inputting a first plurality of optical channels from a first optical communications line to a first port and inputting a second plurality of optical channels from a second optical communications line to a second port of a 2xc3x972 interleaved channel separator; (b) outputting the first plurality and the second plurality of optical channels from a third port of the 2xc3x972 interleaved channel separator to the input of an optical amplifier; (c) outputting the first plurality and the second plurality of optical channels from the output of the optical amplifier to a fourth port of the 2xc3x972 interleaved channel separator; and (d) outputting the first plurality of optical channels to the second fiber optic communications line from the second port of the 2xc3x972 interleaved channel separator and outputting the second plurality of optical channels from the first port of the 2xc3x972 interleaved channel separator to the first fiber optic communications line.